1.2.2 Design Procedure.
18.104.22.168 Selection of Electrode Configuration. Determine what type of earth electrode subsystem is most
appropriate for the facility (complex, building, structure, transformer bank, substation, etc). The directed
configuration is a ring ground outlined in paragraph 22.214.171.124.3 of MIL-STD-188-124A. If this configuration
cannot be employed, alternate configurations meeting these requirements are described in Section 126.96.36.199 of
Establish the primary functional requirements to be met by the earth electrode subsystem. For
Lightning. For a facility located in an area of high lightning incidence or a high degree of exposure
to lightning, or both, (see Volume I, Section 3.4) the earth electrode subsystem must safely dissipate the
lightning energy without melting conductors or overheating the soil (see Volume I, Section 188.8.131.52). Also, the
subsystem must minimize step voltages in areas where personnel are present.
earth electrode subsystem, it must have low rf impedance properties.
Mobility. Mobile facilities or temporary transportable facilities will generally not justify the
installation of an extensive fixed electrode subsystem. For such facilities, install only a basic system capable
of providing the minimum acceptable lightning and personnel fault protection (see Section 1.11).
Resistance. At fixed C-E facilities, the earth electrode subsystem should exhibit a resistance to
earth of 10 ohms or less. If 10 ohms is not economically feasible by the ring ground, alternate methods should
be considered. Paragraph 184.108.40.206.3.2 of MIL-STD-188-124A refers. Resistance measurements using the
fall-of-potential method shall be accomplished in 3-month increments for 12 months following installation.
Measurements shall be conducted in al-month intervals after the first year.
Evaluate local conditions.
Soil resistivity. Is soil resistivity low (< 5000 ohm-cm), average (5000 to 20,000 ohm -cm), or high
(> 20,000 ohm-cm)? The higher the soil resistivity, the more complex (and expensive) will be the electrode
subsystem necessary to achieve 10 ohms resistance.
Moisture content. Is the water table near the surface or far below grade, and is it subject to large
seasonal variations? Design the earth electrode subsystem so that it makes and maintains contact with soil that
stays damp or moist year round if at all possible. Penetration of the permanent water table is highly desirable.
Frost line. How deeply does the frost line extend, even during coldest periods? The resistivity of
soil rises greatly (see Volume I, Section 2.3.3) as the soil temperature drops below 32 F. Thus for maximum
stability of electrode resistance, the subsystem should penetrate far enough into the soil so that contact is
always maintained with unfrozen soil. The earthing techniques described in this chapter are not directly
applicable to permafrost. In permafrost, fault protection must be provided through the use of metallic returns
accompanying the power conductors to insure the existence of a return path to the transformer or generator.